Air pollution what is it?

Air pollution can be either in the form of molcules on the gas form (single molecules) or as particles (more molecules together).

Particles

PM₁ is particulate matter smaller than 1 µm in diameter 

PM_2.5 accordingly 2.5 µm (also designated fine particles)

PM₄ is a special case as this is approximately the cut-off for respirable particles - particles that are small enough to enter the alveoli (where oxygen and carbon dioxide exchange occurs)

PM₁₀ thus 10 µm (also designated coarse particles)

PM_0.1 are also called ultrafine particles and are thus 100 nm or smaller (I would call them nanoparticles)

PM consists both of organic and inorganic molecules. Organic molecules are those that contain carbon while inorganic molecules do not.

Sulphate is the main part of the inorganic fraction of PM₁, others include HNO₃, H₂SO₄ and ammonia. H₂SO₄ and SO₂ contribute to acid rain together with nitric acids

H₂SO₄ and SO₂ stem from the combustion of such as wood, coke and oil (other sources are algae and volcanic eruptions)

OCS (O=C=S) is also abundant in the atmosphere (stems from oceans and combustion)

Gas

Abundant molecules are NO₂ and NO (combined designation NO_x) or ozone (O₃)

Here is a link to an article on the inorganic fraction

What is the size of particles that can reach the alveoli?

Brown et al. estimated (citation) 'For typical activity levels and breathing habits, we
estimated a 50% cut-size for the thoracic fraction at an aerodynamic diameter of around 3 μm in adults and 5 μm in children'

(Particle and Fibre Toxicology 2013, 10:12)  

I think it is highly interesting that the cut-size is hihger in children - suggesting they are more susceptible to air pollution exposure-